Pipe joint and a gasket therefor

ABSTRACT

A gasket  17  for a demountable pipe joint  1  of an hygienic food processing system, the gasket  17  comprising a first part  19  defining a surface  27  for providing a smooth transition from a first pipe end  3  to a second pipe end  5  at a pipe joint  1 , the sides of the gasket  17  each defining a plateau  25  adjacent the surface  27  for abutting the pipe ends  3, 5,  in use, to produce an hygienic seal, wherein the first part  19  of the gasket  17  is made from a plastics material having sufficient structural strength when not fully supported to withstand pressures within the pipes  3, 5,  during use, without damaging or separating from the pipe ends. Such a gasket has been found to be extremely efficient and appropriate in hygienic food processing systems, which in the past have relied heavily on elastomeric gaskets.

BACKGROUND OF THE INVENTION

[0001] Systems and apparatus for the processing of liquid foodinevitably require joints between adjacent pipes of the apparatus.Although pipes can be joined by welding, they can not then be readilydisassembled for cleaning and other purposes.

[0002] In view of the foregoing, demountable pipe joints are preferredin many applications, although it is recognized that such joints (i.e.joints that are not welded together but can be assembled anddisassembled) are unsatisfactory from a hygienic point of view. Problemsassociated with demountable pipe joints are discussed in a paper by theEuropean Hygienic Equipment Design Group entitled “Hygienic Design ofClosed Equipment for the Processing of Liquid Food” by G. J. Curiel, Dr.G. Hauser, P. Peschel and D. A. Timperley dated October, 1993. In thisdocument, various types of pipe joint incorporating elastomeric gasketsare discussed. As a result, in general, it is clear that gaskets,whether in the form of sheets, moldings or “O” rings, which are usuallymade of elastomer such as Neoprene rubber, EPDM or Viton, are not ideal.These known prior art gaskets are used because they deform to take theshape of the mating surfaces when relatively small forces are applied.However, these gasket materials “creep”, particularly at hightemperatures, and compress or move readily during use in response to thepressure of liquid food in the pipe of the processing apparatus.

[0003] If a gasket exhibits “creep” (—creep means a time dependentchange of relaxed dimensions after having been subject to heat and/orpressure), the pressure between the gasket and a mating face of a pipeend at a joint can be lost and crevices can open up. As a result, microorganisms can become established in the crevices, thereby preventing thepipe joint remaining sterile. Furthermore, problems with micro organismscan be especially bad when using elastomeric gaskets because the foodprocessing equipment usually goes through cycles of heating and coolingduring long production runs and the micro organisms can easily flourishin crevices as the soft elastomeric gasket moves in its seat.

[0004] Another problem associated with elastomeric gaskets is that, whena pipe needs to be sterilized before a new production run is commenced,temperatures between 122° and 140° (for example) need to be applied,often by means of superheated pressurized water or steam to kill anymicro-organisms present. If there are crevices present then thesterilizing agent may not contact the entire surface. Heating elastomersto this sort of temperature, however, results in degradation of thematerial, thereby shortening the life of the gasket.

[0005] U.S. Pat. No. 5518257 discloses a seal device which incorporatesan inner seal member made from engineering plastics and resilientlybiased towards a flow path. The inner seal member is designed to sliderelative to pipe ends of a joint and includes a sharp internal corner,both of which are inherently unsatisfactory in hygienic applications dueto their ability to harbor micro-organisms.

[0006] Although metal gaskets are known in certain applications, theseare not considered appropriate in food processing apparatus because theycan result in scars and scratches being produced on the pipe ends which,when a new gasket is applied, will prevent a satisfactory seal beingproduced.

[0007] Although the problems described above have been known for manyyears in the field of hygienic food processing equipment, as far as theapplicant is aware no one has provided a satisfactory answer until now.

BRIEF SUMMARY OF THE INVENTION

[0008] With the foregoing in mind, the present applicant has invented anew demountable pipe joint and gasket suitable therefor which overcomethe problems associated with the prior art in a simple and efficientmanner. Furthermore, although the present invention appears to goagainst all teachings of the prior art in this particular specializedfield, it provides a significant improvement over the prior art.

[0009] According to the present invention, there is provided anapparatus for use in a hygienic food processing system, comprising agasket made substantially of engineering plastics material, wherein saidengineering plastics material is selected to be resistant to creepdining said gasket's use in said hygienic food processing system.According to a preferred embodiment the gasket comprises a bore surfacefor providing a transition from a first joint part to a second jointpart at a demountable joint of said hygienic food processing system, anda side defining a seal surface, adjacent the bore surface, for abuttingat least one of said first and second joint parts, wherein said sealsurface deforms in use to define a plateau positioned on a nib whichextends from a body of the gasket. In use the gasket produces anhygienic seal.

[0010] As will be appreciated, a gasket according to the presentinvention is a significant departure away from known prior art gasketsof the relevant type, which in general were elastomeric and soft.Furthermore, although particularly applicable to hygienic foodprocessing systems, a gasket according to the present invention may beused in any demountable joint or other appropriate application. Forexample, the invention may find significant uses in the biotechnology orpharmaceutical industries where cleanliness is required.

[0011] Although those skilled in the art will understand thesignificance of the present invention, in particular the differencebetween elastomers and engineering plastics, the distinction can beclearly recognized from the differences in relative tensilestrength/modulus. In particular, a typical elastomer has a tensilestrength of 20-40 MPa and an elongation of 300-700% before breaking.This implies a tensile modulus of less than 10-20 MPa. In contrast, atypical engineering plastic has a tensile strength of 100-200 MPa and anelongation of 3-100%, thereby giving a tensile modulus in the range of2400-10,000 MPa at room temperature. Further, a typical elastomerexhibits between 2-3 times as much linear thermal expansion (and hencebetween 8-27 times as much volume expansion) as an engineering plasticssuch as polyetheretherketone.

[0012] Preferably each seal surface is positioned on a nib or platformwhich extends from a body of the gasket. In a particular embodiment,wherein the gasket is substantially annular, each nib is alsosubstantially annular.

[0013] In a preferred embodiment, each seal surface includes adeformable ridge for mating with imperfections in a joint part duringproduction of the hygienic seal.

[0014] In an alternative embodiment, each pipe end may include a raisedplatform portion for gripping a simply shaped gasket, such that theraised platforms of the pipe ends replace the nibs of the gasket.

[0015] The nibs are preferably shaped and sized to exhibit creep duringassembly of a joint, such that deformation of the nibs results in anextremely good seal being produced between the gasket and the adjacentjoint part.

[0016] The body of the gasket is preferably shaped and sized to remainresilient (i.e. elastic) throughout the lifetime of the gasket. Withthis in mind, provided that the compression force applied to the gasketdoes not exceed the critical stress (in this specification, “criticalstress” means the maximum stress at which no relatively rapid permanentdeformation occurs) of the particular plastics material forming thegasket, the body of the gasket can remain resilient such that the nibsof the gasket are continually biased into contact with the joint parts.This preferably applies at all times and at all working temperaturesexperienced by the gasket.

[0017] The radial width of each seal surface of the gasket may be lessthan 50%, preferably less than 22%, more preferably about 20%, of theradial width of the body of the gasket. Further, the axial length ofeach nib is preferably less than 5%, more preferably less than 2%, ofthe total axial length of the gasket. By using these preferred relativedimensions, normal usage of the gasket results in the nibs experiencingcreep during assembly of a joint and the body of the gasket remainingresilient throughout the assembly and subsequent lifetime of the joint,thereby keeping the deformed portion in contact with the joint part andthus the seal tight despite thermal cycling. A markedly improved seal isthereby provided which should not require any subsequent tightening ofthe joint.

[0018] The gasket may include a stop for defining the maximumcompression of the gasket during use. The stop may be made of metal,possibly stainless steel, ceramic or plastics material. If the stop ifmade of plastics, it may be formed integrally with the gasket and takethe form of an annular outer portion of the gasket which definesoppositely facing planar surfaces.

[0019] The gasket may include a metal fence which defines the maximumradial extent of the gasket. If a fence is employed, the fence may be astainless steel ring which abuts the gasket and acts as the stopmentioned above. As a result, the inner plastics part of the gasketcannot expand radially, but can expand axially if necessary. The amountof expansion will, however, be minimal in comparison to the prior artelastomeric gaskets.

[0020] Preferably the sides of the gasket adjacent the fence defineraised surfaces which extend further axially than the fence. As aresult, when the gasket is positioned for use and compressed in a pipejoint, for example, the fence acts as the stop and the plastics part ofthe gasket can only be compressed by a predetermined amount dictated bythe axial height of the raised surfaces.

[0021] An annular trough may be formed on each side of a gasket betweenthe plateau and the raised surface. Alternatively, in use, a cavity maybe defined around this region of the gasket. In any event, in thisregion, the gasket is unsupported and allows expansion of the gasket, ifnecessary.

[0022] The plastics material of the gasket is preferably resistant tocreep at temperatures encountered during use. In particular, theplastics material is preferably resistant to creep up to about 140° C.It is envisaged that a gasket according to the present invention islikely to be used in a pipe joint which may be exposed to sterilizationtemperatures of between 122° C. and 140° C., and may undergo manythermal cycles.

[0023] Preferably the plastics material of the gasket is a hard plastic,such as polyetheretherketone (PEEK), polyethersulfone (PES), TORLON(Trade Mark), a polyamide-imide, or a polycarbonate. Other hard/toughplastics, such as polyphenylsulfone (e.g. RADEL from Amoco), LiquidCrystal Polymer (e.g. VECTRA from Hoechst, or ZENITE from Dupont) orpolyimide (e.g. VESPEL from Dupont), known to those skilled in therelevant art may also be used, if appropriate.

[0024] Although it may ultimately not be the preferred form of gasketaccording to the invention, it is envisaged that the gasket may be madesolely of PEEK or other hard/tough plastics material. Furthermore, sucha gasket may be used not only in a pipe joint, but also in or as part ofa housing or body of a valve, a pump or a heat exchanger, for example.

[0025] According to another aspect of the present invention, there isprovided a pipe joint of an hygienic food processing system comprising afirst pipe end, a second pipe end and a gasket as claimed hereincompressed between the two pipe ends.

[0026] The pipe ends may be urged together by means of flanges solderedonto the outsides of the pipes. By soldering the flanges onto the pipes,welding at the pipe joint can be avoided altogether. As a result, thesignificant temperatures associated with welding are not needed, therebypreventing damage to the pipes themselves, which may be made from highgrade austenitic stainless steel.

[0027] When using flanges soldered on the outsides of the pipes, contactbetween the soldering metals (possibly tin based) and food productspassing through the pipes is best avoided. This can be readily achievedusing a gasket according to the present invention, which does not movesignificantly during use yet provides a reliable seal against materialfrom within the pipes reaching the flanges on the outsides of the pipes.Hence, the advantages of soldering the flanges onto the pipes can berealized.

[0028] Preferably alignment of the pipe ends is controlled by theflanges. As will be appreciated, if the thickness of a pipe wall is onlybetween 1.2 and 1.6 millimeters, it can be extremely difficult to aligna gasket accurately with the pipe ends. Furthermore, producing a goodseal between two thin pipe walls and an elastomeric gasket is extremelydifficult to achieve. The present invention clearly assists inovercoming such problems. With this in mind, an inside surface of one offlanges preferably assists in aligning the gasket.

[0029] In one embodiment, the flanges may be drawn together by means ofa screw thread. Any other appropriate means for closing the pipe jointcan, of course, be envisaged by those skilled in the relevant art.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Specific embodiments of the present invention are now described,by way of example only, with reference to the accompanying drawings, inwhich:

[0031]FIG. 1 is a sectional side view through a de-mountable pipe joint;

[0032]FIG. 2 is a sectional side view showing the gasket used in thepipe joint of FIG. 1;

[0033]FIG. 3 is an enlarged view of the part ringed in FIG. 2;

[0034]FIG. 4 is a view in the axial direction of the gasket of FIG. 2;

[0035]FIG. 5 is a sectional side view of part of a second de-mountablepipe joint;

[0036]FIG. 6 is an enlarged view of the gasket part shown in FIG. 5;

[0037]FIGS. 7a, 7 b, 7 c and 7 d are schematic sectional side views ofpart of a third demountable pipe joint during closing of the joint;

[0038]FIG. 8 is a graph showing the percentage of creep exhibited byPEEK at various temperatures and stresses; and

[0039]FIG. 9 is a seal cross-section with various dimensions and areaslabeled, as discussed in Appendix A.

DETAILED DESCRIPTION OF THE INVENTION

[0040] With reference to FIG. 1 of the drawings, a de-mountable pipejoint 1 of an hygienic food processing system is shown. The pipe joint 1joins the ends of two adjacent flow pipes 3,5. Each pipe carries aflange 7,9 adjacent its end which is soldered to the outside of the pipe3,5. In the embodiment shown, each flange 7,9 includes an outer screwthread 11,13 which mates with a ring nut 15 as shown. Although notnecessarily the preferred way of drawing the two pipe ends together, thering nut 15 can have two threads of different pitches for engaging thescrew threads 11,13 of the two different flanges 7,9. As a result,rotation of the ring nut 15 can draw together or separate the twoflanges 7,9 in an accurate fashion. Other ways of securing the twoflanges 7,9 together, such as by using a chamfered clamping ringtightened by a wing (or butterfly) nut, can of course be envisaged bythose skilled in the relevant art.

[0041] A gasket 17 is positioned between the ends of the pipes 3,5 andthe flanges 7,9. As seen more clearly in FIGS. 2-4, the gasket 17 isannular in shape and comprises an inner ring 19 of polyetheretherketone(PEEK) or any other appropriate plastics material having significantstructural strength so that it does not move significantly as a resultof pressure within the pipes 3,5 during use. The plastics material must,however, be soft enough for it to deform to the metal surfaces of thepipe ends to form hygienic and reliable seals therewith. Furthermore,the plastics material must be very resistant to creep at the temperatureand cycling operations experienced in hygienic food processing systems,namely up to perhaps 140° C. during sterilization. Additionally, thegasket material must be inert to chemical reaction with cleaning andsterilizing materials, and with food products passing through the pipes3,5.

[0042] An outer ring 21 of stainless steel abuts the plastics ring 19,as shown more clearly in FIG. 3. As can also be seen in FIG. 3, theinner plastics ring of the gasket 17 includes raised surfaces 23adjacent the metal ring 21. As a result, during closing of the pipejoint 1, the flanges 7,9 bear against the raised surfaces 23 andcompress the plastics ring 19 to form an hygienic seal before thestainless steel ring 21 acts as a stop against the compression. Thestainless steel ring 21 may also prevent the plastics ring 19 frombursting outwards in the event of an extremely high pressure within thepipes 3,5.

[0043] An annular plateaux 25 is also formed on each side of theplastics ring 19 adjacent an inside surface 27 of the gasket 17. As willbe appreciated, the inner surface 27 allows a smooth transition from theinner surface of the first pipe 3 to the inner surface of the secondpipe 5, as shown in FIG. 1. Further, the two plateau 25 have widthscorresponding to the thickness of each pipe 3,5, so that the pressureapplied to the gasket 17 by each pipe end as the pipe joint 1 closes isfocused onto the corresponding plateau 25, thereby maximizing thesealing pressure and forming an excellent hygienic seal.

[0044] Between the plateaux 25 and the raised surfaces 23, a trough 29is formed in each side of the plastics ring 19. During use, in thisarea, the gasket 17 is unsupported, thereby providing free space intowhich the plastics ring 19 can expand its volume during heating withoutpushing the inner surface 27 (i.e. the product contacting surface) outinto the flow of the product. Although it is true that an additionaldegree of pressure will be applied to the plateaux 25 by the pipe endsand to the raised surfaces 23 by the steel ring 21 due to the fact thatthe plastics will expand to a greater extent than the surrounding metalduring heating, the free space adjacent the plateaux 25 and raisedsurfaces 23 allows the plastics ring 19 to accommodate this additionalpressure without causing premature aging of the material.

[0045] As can be seen in FIG. 1, the flanges 7,9 can be made accuratelyto hold the gasket 17, which is relatively rigid, with a tight fit,thereby ensuring that the gasket 17 aligns accurately with the pipeends. This is a distinct and marked improvement over prior art gasketsusing elastomeric material, which can easily become deformed duringinstallation.

[0046] As can also be seen in FIG. 1, ducts 29,31 are formed in theflanges 7,9. These ducts allow access to the free space 33,35 adjacentthe plastics ring 19 of the gasket 17, thereby enabling the pipe jointto be checked for leaks. Further, sterilizing steam or water, perhaps athigh pressure for example, can be applied to the gasket 17 behind theplateaux 25 to kill off any micro-organisms which happen to accumulatethere. Hence, micro-organism cannot hide from the sterilization processgoing on within the pipes 3,5 simply by hiding behind the hygienic sealformed at the plateaux 25.

[0047] Finally, as can be seen in FIG. 3, the plateaux 25 and raisedsurfaces 23 can be made with essentially the same height, therebyensuring that the two regions are compressed to a similar extent duringclosing of the pipe joint 1.

[0048] Turning now to the second embodiment shown in FIGS. 5 and 6, anannular gasket 100 is held between end faces 102,104 of pipes 106,108.Integral flanges 110,112 are formed at the ends of the pipes 106,108.The flanges 110,112 define chamfered surfaces 114,116 which are engaged,in use, by a clamping ring (not shown) which can be tightened using abutterfly or wing nut to draw the two flanges 110,112 together tocompress the gasket 100 between the pipe ends. The maximum compressionof the gasket 100 is, however, dictated by a mechanical stop which takeseffect when an extension 118 of flange 110 abuts against flange 112, asshown in FIG. 5.

[0049] The gasket 100, which is shown more clearly in FIG. 6, is annularand includes a body portion 120 and two nib (or raised platform)portions 122 for abutting the end faces 102,104 of the pipes 106,108.Each nib portion 122 defines a plateau surface 124 which conforms to anysurface imperfections in the end faces 102,104 of the pipes 106,108during assembly of the pipe joint. More particularly, the plasticsmaterial from which the gasket 100 is manufactured in one piece ischosen such that the nib portions 122 exhibit creep during assembly ofthe pipe joint as the two pipes 106,108 are drawn together by theclamping ring. In contrast, the body portion 120 of the gasket 100remains resilient throughout the assembly of the pipe joint and itssubsequent lifetime, thereby acting as a spring to bias the nib portions122 continually into contact with the end faces 102,104 of the pipes106,108. Hence, although not every hard plastics material can satisfythese requirements, those that can provide an excellent gasket forhygienic use. As stated above, a particularly appropriate plasticsmaterial for this application is polyetheretherketone (PEEK).

[0050] To enable the plastics material of the gasket 100 to act in thedesired fashion, with the nib portions 122 exhibiting creep duringinstallation and the body portion 120 remaining resilient, relativedimensions of the nib portions 122 and the body portion 120 areimportant. In particular, the applicants have found that the radialwidth (C) of each plateau surface 124 must be less than 50%, morepreferably about 20%, of the total radial width (D) of the body portion120. Further, the axial length (A) of each nib portion 122 should beless than 5%, more preferably less than 2%, most preferably about 1.875%of the axial length (B) of the body portion 120. By following theserelative dimensions, the resilience of the body portion 120 canaccommodate the expansion and contraction of the gasket 100 duringheating and cooling of the gasket 100 during normal usage in an hygienicprocessing system without allowing crevices or cavities to open upbetween the plateau surfaces 124 of the nib portions 122 and the endfaces 102,104 of the pipes 106,108.

[0051] The mathematics which explain the allowable changes in relativelength of the body portion 120 and the nib portions 122 is given inAppendix A hereinafter.

[0052] Turning now to FIGS. 7a-7 d, a cross-section of one side of athird embodiment of gasket 200 between pipe ends is shown during closingof a pipe joint. FIG. 7a shows a gasket before compression, FIG. 7bshows the gasket following compression by 0.001 inch, FIG. 7c shows thegasket following compression by 0.002 inch and FIG. 7d shows the gasketfollowing compression by 0.003 inch. As can be seen clearly in FIG. 7a,this embodiment of gasket 200 comprises a bore surface 202 and anadjacent pair of non-flat seal surfaces 204. In fact, each seal surfaceincludes an annular ridge 206 which, whenever compressed, deforms andmates with imperfections in the surface of the adjacent joint part toassist in producing a high quality hygienic seal.

[0053] As can be seen more clearly in FIGS. 7b-7 d, as the pipe ends 208are brought together, each seal surface 204 is compressed and deforms todefine a plateau 210 on a nib 212 which extends from a body portion 214of the gasket 200. More particularly, as the gasket 200 is compressed by0.002 inch, a nib defining a plateau having a width of approximately0.0055 inch is produced in this particular embodiment. As a result, anextremely reliable hygienic joint is formed between the metal pipe endsand the engineering plastics material gasket 200.

[0054] As discussed above, the nibs 212 exhibit creep during formationof the plateau 210. In contrast, the body portions 214 of the gasket 200remain elastic (resilient) throughout, thereby acting as a spring tobias the nib portions 212 (and hence the seal surfaces 204) continuallyinto contact with the end faces of the pipes. This applies even thoughthe pipe joint may be exposed to thermal cycling during use.

[0055] As discussed above, polyetheretherketone (PEEK) is a particularlygood plastics material for a gasket 100 according to the presentinvention. Reasons for this can be appreciated from the graphs shown inFIG. 8 of the drawings. As can be seen in FIG. 8, substantially no creepoccurs at the following temperatures and stresses:

[0056] 114° C. and 45.5 MNm⁻²

[0057] 124° C. and 39.6 MNm⁻²

[0058] 142° C. and 36.0 MNm⁻²

[0059] With this in mind, as mentioned previously, an hygienic foodprocessing system may operate between room temperature and up to about140° C. during sterilization.

[0060] However, if the temperature and/or stress is increased over athreshold, as evidenced by conditions such as

[0061] 124° C. and 45.5 MNm⁻²

[0062] 135° C. and 39.2 MNm⁻²

[0063] 142° C. and 39.3 MNm⁻²

[0064] a significant amount of creep is exhibited over a relativelyshort period of time. Thus, PEEK exhibits a sharp change from an elasticregion with very limited creep to a plastic region with substantialrapid creep. Using these characteristics, the gasket 100 can be designedto allow the nib portions 122 to exhibit creep during assembly of ajoint, thereby producing an excellent seal between the gasket 100 andthe end faces 102,104 of the pipes 106,108, whilst the body portion 120of the gasket 100 remains resilient throughout its lifetime because itdoes Rot exhibit creep to any significant extent.

[0065] It will of course be understood that the present invention hasbeen described above purely by way of example, and that modifications ofdetail can be made within the scope of the appended claims.

What is claimed as new and desired to be protected by letters patent ofthe United States is:
 1. An apparatus for use in a hygienic foodprocessing system, comprising: a gasket made substantially ofengineering plastics material, wherein said engineering plasticsmaterial is selected to be resistant to creep during said gasket's usein said hygienic food processing system, wherein the gasket comprises: abore surface for providing a transition from a first joint part to asecond joint part at a demountable joint of said hygienic foodprocessing system; and a side defining a seal surface, adjacent the boresurface, for abutting at least one of said first and second joint parts,wherein said seal surface deforms in use to define a plateau and ispositioned on a nib which extends from a body of the gasket, which inuse produces an hygienic seal.
 2. An apparatus as claimed in claim 1,wherein the seal surface includes a deformable ridge for mating withimperfections in a joint part during production of the hygienic seal. 3.An apparatus as claimed in claim 1, wherein the engineering plasticsmaterial has a tensile strength of between about 100-200 MPa and anelongation of about 3-100%, thereby giving a tensile modulus in therange of about 2400-10,000 MPa.
 4. An apparatus as claimed in claim 1,wherein the engineering plastics material is selected from the groupconsisting of: a polyetheretherketone (PEEK); a polyethersulfone (PES);TORLON; a polyimide; and a polycarbonate.
 5. An apparatus as claimed inclaim 1, wherein said nib is substantially annular.
 6. An apparatus asclaimed in claim 5, wherein said nib is shaped and sized to exhibitcreep during assembly of a joint.
 7. An apparatus as claimed in claim 6,wherein the body is shaped and sized to remain resilient throughout thelifetime of the gasket.
 8. An apparatus as claimed in claim, 6, whereina radial width of said seal surface is less than 50% of a radial widthof the body of the gasket.
 9. An apparatus as claimed in claim, 8,wherein the radial width of said seal surface is approximately 20% ofthe radial width of the body of the gasket.
 10. An apparatus as claimedin claim 1, wherein said hygienic seal is formed at least in partthrough compression of said nib.
 11. An apparatus as claimed in claim 1,wherein the engineering plastics is resistant to creep up to 140° C. 12.A pipe joint of an hygienic food processing system comprising: a firstpipe end; a second pipe end; and a gasket having a seal surface disposedannular to and adjacent a bore hole of said gasket, wherein said sealsurface is compressed between the first and second pipe ends such thatdeformation of said seal surface defines a plateau on a nib whichresults in an hygienic seal being produced between the gasket and thefirst and second pipe ends, wherein said gasket is made substantially ofengineering plastics material, and wherein said engineering plasticsmaterial is selected to be resistant to creep during said gasket's usein said hygienic food processing system.
 13. A pipe joint as claimed inclaim 12, wherein said seal surface includes a deformable ridge formating with imperfections in a pipe end during production of thehygienic seal.
 14. A pipe joint as claimed in claim 12, wherein the pipeends are urged together by, means of flanges fastened onto the outsidesof the pipes, so that material flowing in the pipe only contacts thepipe walls and the gasket.
 15. A pipe joint as claimed in claim 14,wherein alignment of the pipe ends is controlled by the flanges.
 16. Apipe joint as claimed in claim 14, wherein the flanges are drawntogether by means of a screw thread.
 17. A pipe joint as claimed inclaim 14, wherein an inside surface of a flange assists in aligning thegasket.
 18. A pipe joint as claimed in claim 14, wherein at least onecavity is provided in the flanges into which the gasket can expandduring use, if necessary.